1. Field of the Invention
The present invention relates to a piezoelectric actuator apparatus, and an ink-jet printer provided with the piezoelectric actuator apparatus.
2. Description of the Related Art
Conventionally, piezoelectric actuators, which drive objects by utilizing piezoelectric deformation (also referred to as piezoelectric strain) in the piezoelectric layer, have been widely used in various technical fields. Among those actuators, U.S. Patent Application Publication No. 2005/0219280 (corresponding to Japanese Patent Application Laid-Open No. 2005-289013) discloses a piezoelectric actuator which is usable in ink-jet heads.
The piezoelectric actuator described in U.S. Patent Application Publication No. 2005/0219280 is provided on a flow passage unit of an ink-jet head including a plurality of pressure chambers each communicating with a plurality of nozzles, and applies a pressure to the ink inside the pressure chambers respectively so as to jet ink droplets from the nozzles. More specifically, the piezoelectric actuator of U.S. Patent Application Publication No. 2005/0219280 has a piezoelectric layer (piezoelectric sheet) arranged to cover the plurality of pressure chambers of the flow passage unit, and two types of electrodes (a plurality of individual electrodes and a common electrode) provided on both surfaces of the piezoelectric layer, respectively. The plurality of individual electrodes are provided to overlap the plurality of pressure chambers respectively, and the common electrode commonly faces the plurality of individual electrodes sandwiching the piezoelectric layer therebetween. In this configuration, when a drive device (driver IC) applies a voltage between the individual electrodes and the common electrode, piezoelectric deformation occurs in a plurality of portions of the piezoelectric layer (piezoelectric elements) sandwiched between the plurality of individual electrodes and the common electrode, thereby applying a pressure to the ink inside the pressure chambers.
However, in the piezoelectric actuator described hereinabove, degradation of dielectric strength voltage sometimes occurs in the piezoelectric elements sandwiched between the two types of electrodes. For example, if a voltage is applied between the electrodes in a state that there is a crack in the piezoelectric element, migration may occur and develop along the crack. Further, if a voltage is applied in a state that water or moisture is absorbed in the crack, water tree phenomenon may occur and develop, thereby causing dielectric strength voltage to degrade in the piezoelectric element. Then, such degradation of dielectric strength voltage may be advanced due to continuous voltage application between the electrodes, and may finally cause a short circuit between the electrodes.
The piezoelectric actuator of U.S. Patent Application Publication No. 2005/0219280 is configured such that liquid droplets are jetted from each of the nozzles by a series of processes including applying a voltage to each of the piezoelectric elements as a standby state, releasing the application of the voltage, and applying the voltage again. Therefore, the voltage is also applied to the piezoelectric elements in an unutilized state (not causing jetting of the liquid droplets from the nozzles), that is, in the standby state. In view of this, in the piezoelectric actuator of U.S. Patent Application Publication No. 2005/0219280, in order to restrain the migration phenomenon from developing due to the same voltage application in the standby state, when only black nozzles are utilized to print a text, for example, the voltage is not applied to the piezoelectric elements (individual electrodes) for color nozzles which do not jet liquid droplets, so as to shorten the time of applying the voltage to each piezoelectric element.
In the piezoelectric actuator of U.S. Patent Application Publication No. 2005/0219280, in order to prevent a short circuit from occurring between the electrodes due to the progress of migration, the voltage is not applied to the unutilized piezoelectric elements thereby shortening the application time of the voltage. However, because it is not possible to specify which piezoelectric elements have a defect, it is not possible to individually shorten the application time of the voltage for the piezoelectric elements with the defects.
Further, as described hereinabove, the degradation of dielectric strength voltage in piezoelectric elements caused by migration and the like does not cause a complete short circuit between the electrodes in a short period of time after its occurrence, but it progresses gradually due to a continuous voltage application to the piezoelectric elements after the occurrence. Therefore, from the point of view of improving the product life, it is preferable to take an appropriate measure to prolong the life of the piezoelectric elements before they become degraded in dielectric strength voltage, that is, before they get into a state that a complete sort circuit has occurred between the electrodes and the piezoelectric elements become unusable.